Vacuum type circuit interrupting device with contacts of infiltrated matrix material

ABSTRACT

The specification discloses an electric circuit interrupting device in which the contacts are enclosed in an evacuated enclosure, and in which the contact material comprises a matrix of a low ductility semi-refractory metal infiltrated with a metal of high electrical conductivity. The metals are chosen so that during alternating current arcing (a) the loss of heat by vapourisation of the metals at the surface of a molten anode spot prevents the anode spot rising to a temperature at which substantial electron emission occurs, so that the ability to interrupt a high current arc at the next natural current zero is improved, and (b) precipitation alloying of the metals occurs, with consequent refinement of the matrix structure at the contact surface so that the performance of the interrupter is improved by conditioning of its contacts. Chromium and cobalt are specifically proposed as matrix metals, and copper, silver, and alloys of copper and silver are proposed as infiltrant metals.

llnited States Patent 11 1 Robinson VACUUM TYPE CIRCUIT INTERRUPTINGDEVICE WITH CONTACTS OF INFILTRATED MATRIX; MATERIAL [75] Inventor:Alfred Alexander Robinson,

Stafford, England [73] Assignee: The English Electric Company Limited,Stafford, England [22] Filed: July 15, 1971 [21] Appl. No.: 162,881

' Related [1.8. Application Data [63] I Continuation-impart of Ser. No.871,431, Oct. 24, 1969, abandoned, which is a continuation-impart ofSer. No. 641,881, May 29, 1967, abandoned.

[30] Foreign Application Priority Data 2,137,281 11/1938 Hensel et a1.200/166 C 2,975,256 3/1961 Lee et a1. 200/144 B 3,008,022 11/1961 Lee .l200/144 B 3,042,474 7/1962 Aurand et al.... 75/173 X 2/1968Kreiselm'aier 75/173 X 1451 June 18, 1974 3,514,559 5/1970 Ranheim v200/166 C 3,551,622 12/1970 Takeuchi et a1. 200/166 C FOREIGN PATENTS ORAPPLICATIONS 406,614 3/1965 Japan 200/144 B 403,937 0/1934 Great Britain200/144 B [5 7] ABSTRACT The specification discloses an electric circuitinterrupting device in which the contacts are enclosed in an evacuatedenclosure, and in which the contact material comprises a matrix of a lowductility semirefractory metal infiltrated with a metal of highelectrical conductivity. 1

The metals are chosen so that during alternating current arcing (a) theloss of heat by vapourisation of the metals at the surface of a moltenanode spot prevents the anode spot rising to a temperature at whichsubstantial electron emission occurs, so that the ability to interrupt ahigh current are at the next natural current zero is improved, and (bprecipitation alloying of the metals occurs, with consequent refinementof the matrix structure at the contact surface so that the performanceof the interrupter is improved by conditioning of its contacts. Chromiumand cobalt are specifically proposed as matrix metals, and copper,silver, and alloys of copper and silver are proposed as infiltrantmetals.

14 Claims, 5 Drawing Figures aim-SL153 PATENTEUJUN 1 8 m4 SHEET 1 BF 3FIG.1

FIG.2

INVENTOR Alfred Alexander Robinson Misegades 8c Douglas ATTORNEYSP'ATENTEDJHMB 1914 3,818 163 SHEET 2 BF 3 l VACUUM TYPE CIRCUITINTERRUPTING DEVICE WITH CONTACTS OF INFILTRATED MATRIX MATERIAL Thisapplication is a-continuation-in-part of application Ser. No. 871,431filed Oct. 24, 1969 by A. A. Robinson relating to Electric CircuitInterrupting Devices, which was a continuation-inpart of applicationSer. No..64l,88 l filed May 29, 1967 byA. A. Robinson Contacts, both ofwhich have been abandoned.

The invention relates to vacuum interrupters, i.e., electric circuitinterrupting devices comprising a pair of cooperating contacts in anevacuated enclosure, the contacts being relatively movable betweenclosed and open positions to complete or interrupt anelectric powercircuit under normal load current and fault current conditions.

Because of the vacuum conditions under which areing occurs between thecontacts when they are separated to interrupt current, the mechanism ofarc extinction in vacuum interrupters is somewhat different from themechanism of arc extinction in other types of circuit interrupters inwhich the arcing occurs in a medium such as insulating gas or oil, andthe tendency for the contacts to weld together is very much more severe.Therefore the choice of contact materials for other types of circuitinterrupters and related devices, such as spark gaps, cannot be taken asrelevant to the choice of Contact materials for vacuum interrupters.

Refractory metals, such as tungsten, molybdenum and their carbides, havebeen successfully used for the contacts of vacuum interrupters ofrelatively low current interrupting capability. In particular, suchcontacts have been constituted by a porous matrix of refractory metalparticles metallurgicallybonded together (specifically, a sinteredcompact of such particles), the interstices of the matrix beinginfiltrated with a nonrefractory metal (usually, copper) of lowermelting and boiling points and higher electrical and thermalconductivities. The refractory matrix provides high mechanical strengthand good erosion resistance and, the matrix metal having little tendencyto weld and being of low ductility, also maintains smooth contactsurfaces and consequently high open-circuit dielectric strength. Thepresence of the non-refractory constituent reduces the current choppinglevel, which is excessively high with contacts of refractory metalalone. However, the current interrupting capability of vacuuminterrupters having contacts of such infiltrated refractory matrixmaterial is limited to about kilo-amperes (peak) owing to excessivethermionic emission from the refractory constituent at higher currents,whereas there is a demand for vacuum interrupters of very much highercurrent interrupting capability.

Until the present invention was made, the develop ment of vacuuminterrupters of higher current interrupting capability had departed fromthe use of infiltrated matrix contact materials and had concentrated onthe use of non-refractory alloys (typically copperbismuth) in which amajor constituent metal is alloyed with a minor constituent which formsbrittle films at the grain boundaries between the crystals of the majorconstituent. With this alloy material the excessive thermionic emissionassociated with refractory metals is avoided, and the effects of theexcessive welding tendency and ductility of non-refractory'metals arerelieved by what may be called inter-crystalline weakness enablinginter-contact welds to be easily broken without drawing spikes from thecontact surfaces. However, the inter-crystalline weakness makes thecontacts mechanically very much interior to those employing theinfiltrated matrix material of low ductility. Being mechanically weakthroughout the structure the material does not have the ability of thematrix material to retain contact profile during operation, for contactseparation tends to rupture intercrystalline boundaries both near theoriginal interface and further into the body of the material. Thismechanical weakness also places many-limitations on the mechanicaldesign of the contacts, which ideally is determined by the plasmaphysics of the arc. Furthermore, such materials of high thermalas wellas electrical conductivity do not generally possess an advantage of theabove-mentioned infiltrated matrix material, namely, a current choppinglevel substantially below that of either of the constituents.

In view of the enormous effort that has, on both sides of the Atlantic,been put into developing and exploiting the idea of using non-refractoryalloy material having inter-crystalline weakness, despite its manydisadvantages, it is apparent that a major inventive contribution to thevacuum interrupter art was needed to put the development of high-currentinterrupters back on to the right track leading towards the idealcontact material for both very high current interrupting capability andreasonably low current chopping level.

Such a contribution has been made by the present invention, according towhich there is provided a vacuum interrupter comprising a pair ofcontacts having cooperating contact-making parts each of which isconstituted by a porous matrix of metal particles metallurgically bondedtogether, the interstices of the matrix being infiltrated with anothermetal of lower melting and boiling points and higher electrical andthermal conductivities, wherein at least the bulk of said particlescomprises particles of a low ductility semirefractory metal which has amelting point substantially higher than that of copper but lower thanthat of mo lybdenum and a boiling point not higher than 3,000C and formsa precipitation alloy with the infiltrated metal, but does not otherwiseform alloys therewith, so that, in, surface regions of the contactswhich are melted by arcing, the precipitation alloy forms and, onsubsequent cooling, re-crystallises to re-establish the infiltratedmatrix structure with the particle size of the semi-refractory metalparticles not exceeding 250 microns,

The matrix may include minor amounts of particles of other metals ifdesired (say, up to about 5 percent by weight of the semi-refractorymetal) provided that they are virtually insoluble in, or do notadversely modify the properties of, the major constituents of theinfiltrated matrix.

The term metal is not necessarily limited to elementary metals; itincludes suitable metallic alloys.

By low ductility is meant a ductility that is low in relation to that ofcopper or silver. The ductility of the matrix metal should be low enoughto cause the infiltrated matrix to exhibit a percentage elongation in atensile test of less than 5 percent. The percentage elongations, by wayof comparison, of copper and silver lie in the range of 40 to 50percent.

Theterms melting point and boiling point are defined as evaluated inScientific Foundations of Vac- Lafferty. It will be appreciatedthatthe'boiling point,

being the. temperature at whichthe vapour pressure equals atmosphericpressure (760mm Hg) is only an indirect guide to the materials behaviourin respect of vapour emission in the interior of a vacuum interrupter.However, the general similarity of the vapour-pressure/temperaturerelationship for the materials under consideration enables boiling pointto be used as a convenient figure'of merit in respectof their vapouremitting properties and, consequently, also of their electron emittingproperties which are temperature dependent. g

The American Society. for Metals Handbook, Vol. 1 p. 34, under theheading Definitions Relating to Metal and Metal Working, defines theword sinter as-follows: 1

Sinter to heat a mass of fine particles for a prolonged time, below themelting point, usually to cause agglomeration. Again, the AmericanSociety for Metals Handbook, Vol. 4, p. 455, under the headingSintering," completes a further definition thus: or a compact'may besintered for a short time and then infiltrated with a molten metal oflower melting point". This latter definition clearly indicates thatsintering can produce a porous mass which can be infiltrated withanother metal.

The precipitation alloying property of the two metals means that thematrix metal is soluble to a substantial extent in the infiltrated metalwhen the latter is liquid, but the solid solubility is low.(i.e., notmore than a few percent). Thus the precipitation alloy will be formed incontact surface regions which are melted by arcingbe tween the contacts,and the surface structure will be refined and improved asa result ofre-precipitation on subsequent cooling.

If, as is preferred, the particle size of the matrix metalparticles'does not exceed 250 microns throughout the matrix, theperformance of the interrupter will be'particularly good ab initio.However, it is possible to use a much larger particle size than 250microns in manufacture since precipitation alloying during arcing may berelied upon to refine the surface structure and give improvedinterrupter performance. This may be achieved by a factory conditioningprocess before the interrupter is put into service, or by arcing duringnormal operation of the interrupter in service.

Preferably, the semi-refractory metal comprises chromium, though it mayalternatively comprise, for example, cobalt or an alloy (or solidsolution) consisting substantiallyof desired proportions as they arefully soluble in one another in all proportions. Another possibility isto use a mixture of. particles of more than one such semi-refractorymetal (e.g., chromium particles and cobalt particles) each of whichforms such a pre- 4 cludin'g eithe'r or both of copper and silver withwhich the matrix metal does not form an alloy other than a precipitationalloy. 3

Alloys of copper may include zirconium, tantalum or titanium, thoughonly in small proportions; for example, the alloy-may consist of 99.7per cent copper and 0.3 per cent zirconium by weight. Preferably theinfiltrated metal'occupies between 10 and 35 per cent of the volume ofthe infiltrated matrix, giving particularly low inter-contact weldstrength.

One vacuum interrupter embodying the present invention will now bedescribed by "way of example and with reference to the accompanyingdrawings, in which; i

F IG. 1 showsa vertical cross-section through the interrupter, g

FIG. 2 shows diagrammatically andto a very much larger scale a verticalcross section through an arcing portion of one contact of the vacuuminterrupter, FIG. 3 is a photomicrograph of part of a cross-section of avacuum interrupter contact, showing an arcrefined surface layer on :the,unmodified substrate of a copper-infiltrated chromium matrix, and

- FIG. 4 is a graphical representation of test results showing thevariation with percentage composition of the force required to breakwelds between experimental copper-infiltrated chromium matrix tippedtest pieces simulating vacuum interrupter contacts.

FIG. 4A is a cross-sectional view of a weld test piece.

' Referring to FIG. 1, the'vacuum interrupter comprises a pair of endplates 11, 12 bonded in a vacuumtight manner respectively to cylinders13, 14 of insulating material. The cylinders 13, 14 are bonded to aflange 15 which is trapped between them and carries a shield 16 ofgenerally cylindrically form.

The vacuum interrupter is provided with a pair of relatively separablecontacts 17,18, the movable contact 17 being capable of movement bymeans of an actuator (not shown) towards and away from the fixed contact18. The movable contact 17 has its contact stem 21 reciprocable in abushing 19, and a flexible conductor is provided which is attached tothe contact stem 21. A bellows device .20 is secured-in a vacuum-tightmanner to the contact stem 21 and to the base plate 12 to allow movementof the contact 17.

The contact stem 2l has a contact head 22 secured to it. The latter isrecessed at its centre to afford a flat annular face 23, whichco-operates with a similar face 24 on the cooperating contact 18 whenthe contacts are moved into engagement. The fixed contact 18 has a stem26, which is secured to the base plate 11 and provides the otherterminal of the circuit interrupter, and a head 27 which mayconveniently be symmetrical with that of the movablecontact 17.

The contact heads 22, 27 are manufactured by compactingcommercially-available pure chromium powder which is not excessivelyoxidised (e.g., powder as made by the Thermit process) of particle sizenot exceeding 250 microns, and then sintering the compact under highvacuum. The sintered compact is then infiltrated with molten copper'under high vacuum and at a high temperature (about I,200 C) just abovethe melting point of copper. The copper occupies between 10 and 35 percent of the volume of the infiltrated matrix material, as determined bythe porosity of the sintered compact, and hence as determined by thedegree of compaction applied to the compact. If necessary, theinfiltrated compact may be shaped by normal machining methods. Theductility of chromium is low in relation to that of copper or silver,and the infiltrated matrix exhibits a low ductility giving a percentageelongation of about 2 to 3 per cent where the copper content is 30 percent by volume.

Thus, contrary to the best beliefs of skilled metallurgists havingregard to the substantial solubility of chromium in molten copper (aboutper cent at 1,200 C) which could well have been expected to result indestruction of the matrix structure during infiltration, the presentinventor has succeeded in'bringing together matrix and infiltrant withchromium as the matrix and copper as the infiltrant,

The solid solubility of chromium in copper is low about 0.7 percent nearthe melting point of copper, less than 0.1 percent at 600 C, andasymptotic to 0.05 percent at low temperatures.

In place of chromium, other suitable'low ductility semi-refractorymetals may be used such as, for example, cobalt or'an alloy (or solidsolution) of chromium and cobalt or a mixture of chromium powder andcobalt powder; and in place of copper at number of other metals of goodelectrical conductivity may be used, including silver or an alloyincluding either or both of copper and silver, with which the matrixmetal will not form an alloy other than a precipitation alloy.

Alloys of copper may include zirconium, tantalum or titanium, thoughonly in small proportions; for examplc, the alloy may consist of 99.7per cent copper and 0.3 per cent zirconium by weight.

In FlG. 2 a typical micro-structure of the contact material is shown,the scale marking representing 200 microns, and each chromium particle30 is attached to the neighbouring particles of chromium 30 (shownhatched) and immersed in copper 31 which completely fills theinterstices.

Since the chromium particle size is limited to 250 microns. the maximumsize of asperities is limited, and when such contacts engage underpressure in a vacuum interrupter it is believed that micro-deformationoccurs. resulting in a large number of well distributed contact points.It is considered that this condition would tend to result in lowresistance between the contacts when closed, good distribution of the PRloss at high current flow immediately prior to arcing, and littletendency to weld.

When such vacuum interrupter contacts separate which is effected at highvelocity by actuating means well known per se it is thought that thereis a high probability of multiple arcs being formed between the faces23, 24, giving good distribution of the are energy and consequently lowand uniform erosion.

Even after arcing, it is thought that the size of asperities tends to belimited to that of the maximum dimensions of the particles of thematrix,so that the local electric field stress concentrations and consequentfield emission for'a given contact separation is low, and the breakdownvoltage is higher and more predictable than 'for contacts of similarshapes made mainly of ductile materials.

FIG. 3 of the drawings is a photomicrograph of a secture in the lowerpart of the picture, and the similar but very much refined structure inthe upper part where alloying and re-precipitation has taken place atthe contact surface as a result of arcing. The refined surface layer wasfound to be about 1 millimetre thick. It can be seen that the particlesize of the matrix metal particles in the surface layer is very muchless than 250 microns, in fact only a few microns or less. Thus thearc-conditioned surface layer is very smooth and gives excellentvacuum-interrupter performance in respect of dielectric strength whenthe contacts are open, avoidance of serious contact welding when closedon to a fault, and avoidance of serious erosion due to explosive removalof 'asperities under through fault current and when the contacts are.parted to interrupt the current. By chemical analysis it was found thatthe proportions of chromium and copper in the arced surface layer weregenerally unchanged by the melting process, little preferentialevaporation of copper occurring.

Thus it has been found that arcing between the interrupter contacts hasthe unusual effect of maintaining and even improving the good arcing andvoltage breakdown characteristics of the arcing surfaces and hence thegood performance of the interrupter, instead of the usual effect ofdegrading the arcing surfaces so that in time the performance of theinterrupter falls to an unsatisfactory level.

Moreover, since the boiling point of the semirefractory metal is below3,000 C, electron emission densities following a high-current arcingloop are reduced by several orders below that which occurs with atungsten matrix when surface melting occurs, thus allowing a substantialimprovement to be achieved in the recovery voltage performance.

Tests on contacts made in accordance with the invention from chromiumcompacts impregnated with 30 per cent copper by volume have shown thatarcs up to at least kA peak can be interrupted satisfactorilywith lowand uniform contact erosion and no detectable welding prior to arcing.

As stated above the chromium constituent of the contact material may bereplaced by other metals such as cobalt.

The main properties of each such semi-refractory matrix metal are:

a. it is capable of being wetted by the impregnated metal duringinfiltration;

b. its melting point is higher than that of copper, and (with copper asthe infiltrated metal) is preferably over l,200 C, so that it is notmelted by the infiltration process;

c. its melting point is lower than that of molybdenum;

d. its boiling point is not greater than 3,000 C; and

e. it has low ductility in relation to that of copper and silver.

As stated above the copper constituent of the contact material may bereplaced by other metals such as silver or an alloy-including either orboth of copper and silver.

The main properties of each such infiltration metal are:

1. it has high electrical and thermal conductivity in relation to thatof the matrix metal;

2. its melting point is below that of the semirefractory matrix metaland is preferably below l,200 C;

, jected or subjected to a different degree.

4. it has highductility' relative l-to' that'of the rnatrix metal; v r Iit has low viscosity when'moltenso as to facilitate infiltration'at atemperature just above its melting point. s

with the resultthatfin places some of the matrix, material r rtight'beejected under arcing'conditions or be torn iawayion parting of thecontactsfThe effect offthis would be that'a considerable area of ther'noreductile For reference, the melting points and boiling points i ofthefmetals referred to are given byc'e'rtairiauthorities asfollows,indegrees Centigrade:

1 melting point boiling point a silver (Ag). 1 960.8 1 '2200 v per't-cu)1083 2570 Y.

cobalt (Co) i492 3,000 chromium '(Cr) lSQO 2665 1 molybdenum (Mo) 26254800 3380' 6000 tungsten (W) lt will'be observed that contact materialsaccording to the resentinventiom comprise a relativelysemirefractorymetal of-low ductility and-a'rnetal of relatively goodelectrical and'thermal conductivity;

With such a contactmat'erial the boiling temperature ofthesemi-refractory matrix metal -('e.g.; chromium) issubstantially-belowthe temperature at which appreciable electronemission occurs. It, is believed that when during arcing the temperatureof the anode hot spot approaches or even rises to the point at-whichboth metals boil, the-rate of vapourgeneration increases to a pointatwhich the loss of heat from vaporisation of the metalssubstantially-balances the heatxinput to the hot spot fromthe are, sothat the temperature vof the material would then be present, andthiscould givje rise to the drawing out of. spike formations 'when thecontacts subsequently part, with a consequentialreduction in the voltagewithstand ability of the interrupter under high source voltageconditions."

Degradation of the matrix structure by. alloying, if allowed, would alsohave'an adverseeffect'on the current chopping-performance, in-so far asthereduction of the volume of thematrixmaterial-adjacent-the arcingsurface-relative to that oft'he surrounding infiltrated metal wouldenable more conduction of heat from the arcing surface and so.undesirablyreduce the arcing surface higher current value,

"It hasalso been found thativacuu'r n'interruptersfaccording to thepresent'invention exhibit very 'go od cur I 'rentchoppi'ngcharacteristics, which are, even better than those obtainedwith'interrupters 'havingcontacts made of copper-bismuth alloys orcontacts'of tungsten infiltrated with copper. -It is believed that thisresults from the small size of the metal particles from which thematrix-is=compactedg this size "being'le'ss than thehot spot isheld ato'r nearthe boiling temperature of the jser'ni-refractoryv metaldespite: the presence of higher temperatures in the arc itself. Sincethis boiling temperature of the matrix metal (e.g chrom-iurrrlis.

limit; the anodehot spot temperature-to thatwhich will enable arcextinction at-the next current zeroinstant to hold-off characteristics.Y 3

vlt should be observed that the matrix and infiltrant metals are chosenso that substantially no alloying other thanprecipitation alloying takesplace-between them, the matrix metal-having low solid solubilityin theoccur, and thereby provide good voltage recovery and high current valuesand to infiltrated metal. This is desirable since his desired topreserve, so far as possible, the principal characteristics of thetwoconstituent metals. As the'prior art has shown, alloying. of minor.constituent metals with a majorconstituent metal results in asubstantial modification of the properties of'the major constituent.lnthe present context alloying would result in-an undesirable lack ofuniformity in the propertiesfof the electrodematerial, in that partspreviously subjected toarcing would have different properties from partsnot so subln particular,- alloying of the matrix metal with theinfiltrate d metal would, if allowe d, 'undesi rably increase theelectrical resistance of the infiltrated metal. It

would also tend to weaken the strong matrix structure.

particles 'of the matrix in the higher electrical conducti'vity-infiltrated metaljarid the poor thermal conductivity of thematrix metal which conducts little heat away fromthe arcingsurfaceadjacent 'a cathode spot.

v.This achievement of low chopping current levels-is inclearcontradi'stinction to the technique proposed in the prior art forreducing current chopping. levels by providing in the cont'act'materialsmetals having ahigh .vapouripress'ure, such as antimony,-.bismuth, andtin. However, if azvery low current chopping level is required analloyof copperuwithl percentof, silver can beused as the infiltrated'rnetalmv :lndesigning contacts for a vacuum interrupter ac-. cording ,to thepresent invention care must-be takento keep the proportions of thematrixand infiltrant metals withinreasonable limits. If the contact materialhasv too much matrix metal the voids in the'c ompactedmatrix will beinadequateto give effective infiltration ,of the infiltrant metal, andas a result not only will the contact material have too high anelectricaland thermal resistance, butthere will be aninsufficient-supply of vapour released.from'theacontact material tosustain the arc at' ensure good current" chop! ping performance. v a r vIf on the other hand the contactmaterial'has too little matrix metal theContact material will be physically weak, and there will beinsufficientof the lowconductivity, metal adjacent thearcing surface so that thecurrent chopping performance) will be poor. Furthermore,

the arcing surface will have a high proportion of the more ductilematerial so that the arcingsurfaces will be degraded by the successiveopening of the contacts since spike formations will be drawn fronithecontact surfaces. Hence the voltage withstand performance will be poor 1a i FIG. 4 of the drawings shows the results of weldbreak tests on testpieces as illustrated in that figure,

having tips of copper-infiltrated chromium matrix material similar tothat of a-vacuum interrupter in accordance with the present invention.It illustrates the excellence of the characteristics of the material inthis respect where the infiltrated matrix contains 70 percent or more,by volume, of chromium (i.e., 30 percent or less, by volume, of copper).I

The performance of vacuum interrupters having contacts ofcopper-infiltrated cobalt matrix material is believed to be just as goodas that which has been described with particular reference tocopper-infiltrated chromium matrix material.

I claim:

1. A vacuum interrupter comprising a pair of contacts having cooperatingcontact-making parts each of which is constituted by a porous matrix ofmetal particles metallurgically bonded together by sintering under highvacuum, said metal particles also being defined as metal particlesselected from a group consisting essentially of chromium, cobalt, analloy of chromium and cobalt, and a mixture of chromium powder andcobalt powder; the interstices of the matrix being infiltrated underhigh vacuum with another metal of lower melting and boiling points andhigher electrical and thermal conductivities, said another metal alsodefined as a metal selected from a group consisting essentially ofcopper, silver, an alloy including at least one of the group consistingessentially of copper, silver, an

alloy including at least one of the group copper and silver with whichthe matrix metal does not form an alloy other than a precipitationalloy, and an alloy of copper including at least one of the groupconsisting essentially of zirconium, tantalum and titanium; theinfiltrated metal constituting between 10 and 35 percent of the volumeof the infiltrated matrix and the matrix constituting between 65 and 90percent thereof, wherein at least the bulk of said particles comprisesparticles of a low ductility semi-refractory metal which has a meltingpoint substantially higher than that of copper but lower than that ofmolybdenum and a boiling point not higher than 3,000 C and forms aprecipitation alloy with the infiltrated metal, but does not otherwiseform alloys therewith so that, in surface regions of the contacts whichare melted by arcing, the precipitation alloy conium.

forms and, on subsequent cooling, recrystallizes to reestablish theinfiltrated matrix structure with the particle size of thesemiwefractory metal particles not exceeding 250 microns.

2'. A vacuum interrupter according to claim 1, wherein the infiltratedmetal constitutes about 30 percent of the volume of the infiltratedmatrix.

3. A vacuum interrupter according to claim 1, wherein thesemi-refractory metal comprises chromium.

4. A vacuum interrupter according to claim 3, wherein the infiltratedmetal comprises copper.

5. A vacuum interrupter according to claim 3, wherein the infiltratedmetal comprises silver.

6. A vacuum interrupter according to claim 3, wherein the infiltratedmetal comprises an alloy of copper and silver.

7. A vacuum interrupter according to claim 3, wherein the infiltratedmetal comprises an alloy consisting substantially of 99.7 percent and0.3 percent zir- 8. A vacuum interrupter according to claim 3, whereinthe infiltrated metal constitutes about 30 percent of the volume of theinfiltrated matrix.

9. A vacuum interrupter according to claim 1, wherein thesemi-refractory metal comprises cobalt.

10. A vacuum interrupter according to claim '9, wherein the infiltreatedmetal comprises copper.

11. A vacuum interrupter according to claim 9,

. wherein the infiltrated metal comprises silver.

12. A vacuum interrupter according to claim 9, wherein the infiltratedmetal comprises an alloy of copper and silver.

13. A vacuum interrupter according to claim 9, wherein the infiltratedmetal comprises an alloy consisting substantially of 99.7 percent copperand 0.3 percent zirconium.

14. A vacuum interrupter according to claim 1, wherein thesemi-refractory metal is an alloy comprising chromium and cobalt.

2. A vacuum interrupter according to claim 1, wherein the infiltratedmetal constitutes about 30 percent of the volume of the inFiltratedmatrix.
 3. A vacuum interrupter according to claim 1, wherein thesemi-refractory metal comprises chromium.
 4. A vacuum interrupteraccording to claim 3, wherein the infiltrated metal comprises copper. 5.A vacuum interrupter according to claim 3, wherein the infiltrated metalcomprises silver.
 6. A vacuum interrupter according to claim 3, whereinthe infiltrated metal comprises an alloy of copper and silver.
 7. Avacuum interrupter according to claim 3, wherein the infiltrated metalcomprises an alloy consisting substantially of 99.7 percent and 0.3percent zirconium.
 8. A vacuum interrupter according to claim 3, whereinthe infiltrated metal constitutes about 30 percent of the volume of theinfiltrated matrix.
 9. A vacuum interrupter according to claim 1,wherein the semi-refractory metal comprises cobalt.
 10. A vacuuminterrupter according to claim 9, wherein the infiltreated metalcomprises copper.
 11. A vacuum interrupter according to claim 9, whereinthe infiltrated metal comprises silver.
 12. A vacuum interrupteraccording to claim 9, wherein the infiltrated metal comprises an alloyof copper and silver.
 13. A vacuum interrupter according to claim 9,wherein the infiltrated metal comprises an alloy consistingsubstantially of 99.7 percent copper and 0.3 percent zirconium.
 14. Avacuum interrupter according to claim 1, wherein the semi-refractorymetal is an alloy comprising chromium and cobalt.